An image can be made more appealing by purposely modifying colors in the image. This technique will be referred to as color enhancement hereinafter. For example, relatively pale skin colors, which may faithfully represent a human face, can be shifted towards a preferred skin color by means of image processing. This type of image processing is generally referred to as skin tone shift. As another example, an image, which may faithfully represent green vegetation, can be made more appealing by enhancing saturation of green colors. This type of image processing is generally referred to a green enhance. Another type of color enhancement is a so-called blue stretch, which can make an image with blue sky and white objects, such as clouds, look more appealing.
Color enhancement is preferably carried out in a so-called YUV color space. The YUV color space is a three-dimensional space with a luminance axis “Y”, a first chrominance axis “U”, and a second chrominance axis “V”. A pixel, which has a given color, represents a point in the YUV color space. The pixel can also be regarded as a vector that extends from the origin of the YUV color space to the aforementioned point. The pixel has a luminance component, which corresponds with a position on the luminance axis “Y”, a first chrominance component, which corresponds with a position on the first chrominance axis “U”, and a second chrominance component, which corresponds with a position on the second chrominance axis “V”. The given color of the pixel can be modified by modifying at least one chrominance component. That is, the given color of the pixel can be modified by modifying the respective positions on one of the chrominance axes, or on both axes.
There are relatively many applications, in which an image is available in a so-called RGB format. That is, the image is expressed in an RGB color space, which has a red axis, a green axis, and a blue axis. A pixel of such an image has a red component, a green component, and a blue component. The pixel constitutes a point in the RGB color space whose location is defined by the aforementioned components. The pixel can also be regarded as a vector that extends from the origin of the RGB color space to the aforementioned point.
An image, which is expressed in the RGB color space, may be converted into the YUV color space for the purpose of color enhancement. An YUV image, which is obtained by the aforementioned conversion, is processed in accordance with a type of color enhancement that is desired, such as, for example, skin tone shift. Accordingly, an color-enhanced image is obtained, which is expressed in the YUV color space. An inverse conversion, from the YUV color space to the RGB color space, is necessary in order to obtain a color-enhanced image in the RGB format. U.S. Pat. No. 5,384,601 uses such an approach.